On a conventional turboprop, the propeller is driven by a gas turbine (a free turbine or a linked turbine) via a gearbox, which gearbox is placed between the propeller and the engine and may be of various types: a train of simple gears, a train of compound gears, planetary, epicyclic, etc.
The propellers of such turboprops are provided with a deicing system serving to detach ice that accumulates on the blades, and for that purpose the system requires power of several kilowatts to be delivered by a three-phase alternating current (AC) network, since the low voltage direct current (DC) or single-phase AC power supplies of aircraft are specifically preferred for loads of smaller power. Most present-day deicing systems are electrical systems. Electric heater mats fastened to the pressure side and to the suction side of the propeller, in the vicinity of the leading edge, serve, when powered electrically, to create heat that enables ice that has been formed on the propeller to become detached, which ice is then ejected by centrifugal force.
For this purpose, it is necessary for the mats that are fastened on the blades of the propeller to be powered electrically via equipment that enables electrical power to be transferred from a stationary portion, i.e. the engine, to a rotary portion, i.e. the propeller.
Conventionally, such rotary transfer of electrical power is performed using a slip-ring and brush set, with transfer taking place by contact between firstly the slip-ring(s), mounted on the rear cover of the propeller and constituted by one or more tracks of conductive material, typically copper, and secondly brushes mounted on the engine gearbox and made of conductive material that rubs against the copper track(s). A variant of such an assembly is shown in patent EP 2 730 506 granted to the supplier Hamilton Sundstrand, with an assembly of pieces of equipment needed for rotary transfer (brushes and slip-ring) located at the rear of the gearbox and not between the gearbox and the propeller as is conventionally done.
Nevertheless, that solution presents numerous drawbacks associated essentially with the friction of the brushes against the slip-ring tracks, which is a major source of brush wear. That rapid wear therefore requires regular maintenance operations followed by replacement, which, if disregarded, can have the consequence of making the system unreliable.
Furthermore, the brushes are exposed to splashes of oil, particles of dust (including external particles such as sand, etc.), that can generate electric arcs at the points of contact, which might initiate combustion of inflammable elements constituting the rear panel of the propeller (which may contain magnesium), thereby starting a fire that could lead to the loss of the engine.
It is also known, from U.S. Pat. No. 5,572,178, to incorporate a single-phase rotary transformer on the drive shaft of an aircraft, in order to achieve rotary transfer without contact for the purpose of powering an electrothermal or electromechanical deicing system of medium power (in the range 300 watts (W) to 500 W per blade). Nevertheless, such a solution is not appropriate for purely electrical systems that require electrical power to be transferred at about 1 kilowatt (kW) per blade.
Specifically, under such circumstances, it is necessary to have recourse to a three-phase transformer, which amounts to arranging three single-phase rotary transformers side by side, giving rise to the following drawbacks:                the transformer as built up in this way presents considerable length, making it difficult to integrate on a turbine engine shaft without changing its dimensions, thereby having an overall impact on the length of the engine (implying an increase in weight and difficulties of integrating in the airplane);        the weight of such a transformer is necessarily high, and increases with increasing inside diameter; and        when mounted at the end of a shaft, i.e. when “cantilevered out” on a shaft that may be subjected to considerable forces, it makes the dynamics of the shaft line more complex, since it is necessary to guarantee an airgap that is small, typically less than 1 millimeter (mm).        